In spite of more fiber being deployed in the telecommunications industry, communication lines consisting of a twisted pair are still the most common for delivering signals to customer's premises. This is true for both voice and high speed data.
One common measure of the quality of the twisted pair line is the longitudinal balance of the line. The longitudinal balance is a measure of how well the line rejects external noise. Such noise may come from several sources such as power influence from external power sources, cross talk from adjacent pairs in the cable, and external radio interference (which is more important with high speed data communication).
To understand how longitudinal balance is measured, a short bit of background is necessary. The twisted pair line is a 3 terminal device: (1) the “Tip” is one wire in the pair, (2) the “Ring” is the other wire in the pair, and (3) the “Shield” surrounding the cable. The longitudinal balance is how well matched the impedance between Tip and Shield is compared to the impedance between Ring and Shield.
The basic method of measuring the balance of a line is outlined in the Institute of Electrical and Electronics Engineers (IEEE) Standard 455. FIG. 1 is from the IEEE Standard 455. A balanced circuit (102) internal to a test instrument (104) is connected to twisted pair line in a bridge configuration—i.e., one terminal is connected to the Tip (105) and one to the Ring (110). The test equipment then sends an AC signal (115) onto the Shield of the cable (i.e., a common mode signal in generic engineering terms or a longitudinal signal in industry terms). Any impedance mismatch between the Tip side of the line and the Ring side will result in a signal appearing between Tip and Ring (120) (i.e., a differential signal in generic engineering term or a metallic signal in industry terms). The longitudinal balance is given by the following equation:
            V      m        /          V      s        ⁢          ⁢  or  ⁢          ⁢            (                        V          t                -                  V          r                    )        /          V      s      
In more generic engineering terms (known as the common mode rejection ratio), this equation can be expressed as:
            V      differential        /          V              common        ⁢                                  ⁢        mode              ⁢          ⁢  usually  ⁢          ⁢  measured  ⁢          ⁢  in  ⁢          ⁢  dB
It is known that achieving the best balance on the circuit under test (i.e., the lowest longitudinal balance or the lowest common mode rejection ratio) is limited by the test equipment—specifically by the test equipment's internal balanced circuit. Ideally, the internal balanced circuit is perfectly balanced, that is the impedance presented to the Tip is exactly the same as the impedance presented to the Ring. Said another way with reference to FIG. 1, Z1=Z2.
In practice, however, the internal balanced circuit is not perfectly balanced. Rather, it is a network of resistor, capacitors, and sometimes inductors that have impedance. The internal balanced circuit of most practical instruments includes series capacitors to block any DC current flow, which allows for the testing of lines that are connected to central office equipment. Making the series capacitance as large a possible reduces the impedance of the capacitors, and therefore minimizes their effect on the balance of the internal balanced circuit. There is a practical limit to this however; physical size, expense, and ability to withstand high voltage that sometimes occurs on lines in service limit the amount of capacitance that can be used in a practical device.
Currently in most instruments the series capacitors are hand matched and trimmed, which is labor intensive and time consuming. An example of one instrument that requires hand matching of the series capacitors is U.S. Pat. No. 5,157,336 at 6:5-6 where the capacitors “ . . . are selected in a manner know to one skilled in the art . . . .” Even after hand-matching and trimming the capacitors, their capacitance will drift over time and temperature. Also, each capacitor usually drifts at different rates than the others used in the internal circuit such that it may be impossible to maintain an acceptable level of balance in the internal balanced circuit.
U.S. Pat. No. 5,436,953 by Nilson discusses some of the problems with trying to maintain the precision of an internal balanced circuit. Nilson teaches a method of mathematically correcting for the imbalance of the internal balanced circuit by measuring the balance of the cable in “at least two different connection profiles.” Because the Nilson method requires a relay switch for every measurement, the method works best when only a few measurements must be taken daily—e.g., central office equipment. However it is not well suited to a portable test instrument intended for trouble-shooting which needs to make continuous measurements at rates at least several times per second. Relays would slow down the measurement and wear out quickly. The typical lifespan of a relay is 100,000 operations, which is less than 28 hours of operation at one switch per second.
What is needed therefore is a circuit and method that quickly and efficiently compensates for the internal imbalance and has a long operational lifetime.